A conventional starter for starting an internal combustion engine of a vehicle (patent document 1: JP 11-30139A) is configured to be switchable between two states irrespective of operation/non-operation of its motor. In one state, a pinion gear driven to rotate by the motor is engaged with a ring gear of the engine. In the other state, the pinion gear is not engaged with the ring gear. This starter is referred to as an independently-controlled starter, since the pinion gear and the motor are controllable independently.
Specifically, in an independently-controlled starter 1 exemplarily shown in FIG. 9, a pinion gear 2 is driven to rotate by a starter motor (motor for a starter) 4 under a state that it is engaged with a ring gear 3 of an internal combustion engine (not shown) so that the engine is cranked by rotation of the ring gear 3. This type of starter 1 is provided with a solenoid (pinion control solenoid) 5 and a power supply relay 6 separately. The pinion control solenoid 5 drives the pinion gear 2 for engagement with the ring gear 3. The power supply relay 6 supplies power to the starter motor 4 to for driving the motor 4.
In the field of electric technology, a coil of a solenoid is often referred to as a solenoid. However, in the following description, it is referred to such that a solenoid means an actuator, which includes a coil and a movable part operated by electromagnetic force of the coil. The power supply relay 6 is a relay of large current capacity and has a coil 6a and a pair of fixed contacts 6b and 6c. When a current is supplied to the coil 6a from a battery (power source) 7, the contacts 6b and 6c are shorted to the on-state by a movable contact to supply a current to the motor 4 from the battery 7 through the contacts 6b and 6c. 
It is generally necessary to supply a relatively large current to each of the coil 5a of the pinion control solenoid 5 and the coil 6a of the power supply relay 6. The coils 5a and 6a are thus supplied with currents through two relays, a pinion drive relay RY1 and a motor drive relay RY2, respectively.
More specifically, one end of the coil 5a of the pinion control solenoid 5 and one end of the coil 6a of the power supply relay 6 are connected to a ground line in a vehicle (generally, vehicle chassis). The pinion drive relay RY1 is provided at an upstream (positive) side of the coil 5a and the motor drive relay RY2 is provided at an upstream (positive) side of the coil 6a. Through the relays RY1 and RY2, a battery voltage (voltage of the battery 7) VB is supplied as a power source voltage to the upstream sides of the coils 5a and 6a, which are opposite to the ground line, so that the currents are supplied to each of the coils 5a and 6a. An electric power supply circuit is thus formed in the vehicle.
One end (positive side end) of each of coils L1 and L2 of the relays RY1 and RY2 is connected to a line 8 of the battery voltage VB. An electronic control circuit 9, which controls the starter 1, is provided with transistors T1 and T2. The transistor T1 is for switching over connection and non-connection between the other end (negative side end) of the coil L1 and the ground line. The transistor T2 is for switching over connection and non-connection between the other end (negative side end) of the coil L2 and the ground line.
By turning on the two transistors T1 and T2 in the control circuit 9, the relays RY1 and RY2 are turned on to supply the currents to the coil 5a of the pinion control solenoid 5 and the coil 6a of the power supply relay 6 from the relays RY1 and RY2, respectively, so that the pinion gear 2 is driven to engage with the ring gear 3 and the motor 4 is driven to rotate. The engine is thus cranked by the starter 1.
According to the circuit configuration of patent document 1, the starter motor is supplied with the current through one relay controlled by a signal produced from the control circuit. However, since a large current is supplied to the starter motor 4 in practice, the power supply relay 6 of large current supply capacity is provided inside the starter 1 as shown in FIG. 9. The current is supplied to the coil 6a of the power supply relay 6 through the relay RY2, which is controlled by the control circuit 9.
The patent document 1 also discloses an engine automatic stop and start system (generally referred to as an idle-stop or idling-stop system), which automatically stops an internal combustion engine in a predetermined stop condition and thereafter automatically start the engine in a predetermined start condition. In a vehicle, which is provided with the idle-stop system and referred to as an idle-stop vehicle, it is likely that an independently-controlled starter is used. According to the independently-controlled starter, it is possible to control a pinion gear to be engaged with a ring gear of an internal combustion engine before starting of a starter motor for example, so that wear of mechanical parts such as the pinion gear is reduced and prolong life of the starter. The independently-controlled starter is therefore suitable for the idle-stop vehicle.
In the control circuit 9, which is exemplified in FIG. 9, if an on-failure (continuation of on-state) of the transistor T1 arises, the pinion drive relay RY1 continues to be turned on and the pinion gear 2 continues to be engaged with the ring gear 3. This causes wasteful electric power consumption. Further, since the pinion gear 2 is continuously rotated by drive force of the engine, the pinion gear 2 and other parts such as a one-way clutch provided in the starter 1 wear. The one-way clutch is provided to prevent the motor 4 from being rotated by the ring gear 3 even when the pinion gear 2 is rotated by the ring gear 3 under a state (non-operation state) that no current is supplied to the motor 4. In addition, if an on-failure arises in the transistor T2, the motor drive relay RY2 continues to be turned on and the motor 4 continues to operate. It is thus likely that the motor 4 overheats and becomes inoperative in addition to wasteful power consumption.